Project Summary At the earliest stages of the visual system, signals diverge into separate channels, allowing for parallel processing of visual information. In the mouse retina, 13 retinal bipolar cell types convey visual signals to ~40 retinal ganglion cell types. Traditionally, each neuronal type was thought to convey a single channel of information; however, this view has been challenged by evidence that retinal bipolar cells of nonmammals are able to perform subcellular computations, enabling them to send different signals from different terminals of the same cell. Functional divergence at the level of the synapse has not been shown in the small, relatively electrically compact bipolar cells of the mammalian retina. My preliminary data suggests that different terminals of the same type 6 bipolar cell can transmit different functional signals onto two different retinal ganglion cell types in the mouse. I use confocal imaging to show anatomical connectivity between the type 6 bipolar cell and two different retinal ganglion cells (ON alpha and PixON RGCs). To provide evidence that both the ON alpha and PixON RGCs receive functional input from the same bipolar cell, I show that these cells have high cross correlation of physiological noise. Using voltage clamp recordings and visual stimulation, I show that the type 6 bipolar cell provides excitation with very little surround suppression to ON alpha RGCs but provides excitation with substantial surround suppression to PixON RGCs. Furthermore, I show using dynamic clamp recordings and simulation of recorded conductances that the difference in surround suppression of excitation is what drives the difference in the receptive field properties of the two cells. To explore the circuit and cellular mechanism of this functional divergence, I use pharmacological blockade of receptors and channels to suggest that the type 6 bipolar cell is receiving presynaptic inhibition from a GABAergic spiking amacrine cell that is selective for those type 6 bipolar terminals that provide input to the PixON RGC. To further investigate the anatomy of amacrine cells present at these terminals, I will use serial electron microscopy. Finally, since glutamate release from bipolar cell terminals is calcium dependent, I will image calcium signals from the type 6 bipolar terminals to directly observe divergence of signals at the type 6 terminals. These findings and proposed experiments indicate that each terminal of a single bipolar cell could potentially carry a unique visual signal. This expands the number of visual channels in the inner retina allowing for increased parallelism at the earliest stages of visual processing.